While you don't need to understand what z-code is, exactly, it may help if I explain a bit. Then again, it may only confuse you further: if so, ignore this comment.
You remember text adventures, right? Sometimes they're called "interactive fiction" or "IF". Games where instead of graphics, there's a textual description of your surroundings, and you type commands like "north" or "open door with key" or "attach wheel to rubber chicken then fit rubber chicken to rope then death-slide down ravine"? If you have no idea what I'm talking about, have a go at Hamlet - The Text Adventure
, which runs in a browser and so doesn't require you to install anything.
Back in the day, this was an extremely popular game format, and even today there's an active community of people writing them - because there are no graphics or sound effects to suck up development time, you can spend much more writing time devising elaborate/fun/twisted/etc puzzles for the player to solve. Anyway, back when commercial IF was a going proposition, there were many more popular types of computer than there are now, and porting games to run on many different types of machine was a major headache for the games companies. So one games company, called Infocom, decided that they'd distribute all their games in a machine-neutral format, called z-code (after the first game to use it, Zork). Then they only needed to write one z-code interpreter for each type of computer, and then every game they wrote would be able to be played on every type of machine.
Sadly, Infocom folded some time later, but since then other hackers have written z-code interpreters for more modern computers, so we can continue to play the old Infocom games. There are also programs that allow you to write games in z-code (which can then be played using any of the z-code interpreters), and this is mostly how the modern IF community writes and distributes its games.